Because even if he did it for the greater good and I understand him. It was against her wishes. But this was a turning point for Damon. He learnt from it. He understood what he had done right away and tried to fix it. He even put himself in danger in the process. Elena saw that. She forgave him. They have a clean start and Damon has learned something.

This because he did it out of anger . He gave her his blood out of love. But it helped there relationship because it proved that even killing her brother couldn't keep her hating him

i think elena will forgive what he did in ep20 easier.
1) he did it out of love
2) it happened to elena, i think elena cares about jeremy more than she do for herself.

which ever one it is the bottom line is they will always 4give each other. they have both done things to each other damon killing jeremy was the worst ( thank god he really didnt die) but they keep finding their way back 2 forgiveness... back 2 each other. that's understanding and compassion. that's LOVE

Damon, why have you done this? I know he was sad and disappointed but this was really bad for their relationship!

for damon forcing elena to drink his blood : he want to SAVE her!!

This of course! The fallout from this act of impulse and the consequences of this travesty are what I believe, set the delena story truely into motion.